The present invention relates to communication equipment, and, more particularly, to a receiver portion of a telephone.
The signal received by a telephone of a digital telephone network is processed in digital form, converted into analog form, and finally amplified and applied to an electroacoustic transducer. The receiver portion of a mobile phone can be represented schematically as shown in FIG. 1. A demodulated digital signal RX-IN produced by a receiving and demodulation unit 10 is applied to the input of a circuit unit 11 which converts the signal into analog form, and which filters and amplifies the signal. The analog signal output from the unit 11 is applied to the input of a power amplifier stage which can transfer the signal to an electroacoustic transducer 13, such as an electromagnetic unit or loud speaker, with sufficient electrical power for the operation thereof.
The power amplifier stage comprises an operational amplifier 16 and has a gain determined by the ratio between the resistance of a feedback resistor R2 disposed between the output and the inverting input of the operational amplifier 16 and the resistance of a resistor R1 disposed between the output of the unit 11 and the inverting input of the operational amplifier. To prevent power dissipation when the telephone is not in use, a circuit which responds to an external activation or deactivation control is provided both in the unit 11 and in the amplifier 16. This function is represented by the connection of the unit 11 and of the amplifier 16 to a terminal to which a single-bit digital signal PD (power down) can be applied. By convention, the receiving circuit unit is put in the zero current-consumption state when PD=1, and is enabled to consume current from the supply (power up) when PD=0.
As is known, each time there is a change from the xe2x80x9cpower downxe2x80x9d state to the xe2x80x9cpower upxe2x80x9d state or vice versa, both the signal-processing unit and the power amplifier are subject to an abrupt voltage and current transient before reaching the steady operating state. During this transient, electrical interference with frequency components in the acoustic band are produced and are translated into audible and annoying noises in the electroacoustic transducer.
In an attempt to address these problems, various measures which provide for the connection of active and passive components in series and/or in parallel with the transducer have been adopted to filter out the undesired electrical interference. However, approaches of this type require fairly bulky components outside the power amplifier, which is normally in the form of an integrated circuit.
It has also been proposed to use processing units and amplification units with balanced inputs and outputs to render the electroacoustic transducer insensitive to at least some of the possible interference, particularly so-called common-mode interference. Naturally, however, the use of a balanced amplification unit is possible only when the electroacoustic transducer can be connected between the two balanced outputs of the amplification unit. When this is not possible because one of the terminals of the transducer has to be connected to a fixed power terminal, for example, to the ground of the circuit, the only suitable prior art approach is to use the above-mentioned external filters.
In view of the foregoing background, it is therefore an object of the present invention to provide a receiver portion free of the above-described interference in which the electroacoustic transducer has a terminal connected to the ground terminal, but which does not require filtering components.
This object is achieved by the provision of the receiver portion comprising a differential amplifier stage having differential inputs and having an output, and an electroacoustic transducer connected between the output of the differential amplifier stage and a supply voltage reference. The differential amplifier stage may comprise an operational amplifier having an output defining the output of the differential amplifier stage and having first and second inputs, first and second capacitors connected between respective first and second inputs of the operational amplifier and the differential inputs of the differential amplifier stage, a third capacitor connected between the first input and the output of the operational amplifier, and a fourth capacitor connected between the second input of the operational amplifier and a first reference voltage. In addition, the differential amplifier stage may include a fifth capacitor, and at least one first switch for alternatively connecting the fifth capacitor between a second reference voltage and a third reference voltage, or between the first input and the output of the operational amplifier. The differential amplifier stage may also include a sixth capacitor, and at least one second switch for alternatively connecting the sixth capacitor between a fourth reference voltage and a fifth reference voltage, or between the second input of the operational amplifier and the fifth reference voltage.
The receiver may also include a switching unit for interrupting a connection to the differential inputs of the differential amplifier stage responsive to a control signal. In one embodiment, the switching unit may include at least one third switch responsive to the control unit for connecting the differential inputs to one another at a common terminal for a predetermined time based upon activation of the differential amplifier stage. In another embodiment, the switching unit comprises at least one third switch responsive to the control unit for interrupting a connection between the differential inputs of the differential amplifier stage and respective ones of the first and second capacitors, and for connecting the first and second capacitors to one another at a common terminal for a predetermined time based upon activation of the differential amplifier stage.